I may have misused the term zombie process. Seems that's when an entry for a terminated process doesn't clear it's entry in the process table.

I'm my experience Selenium will appear to be functioning normally from the outside, but the driver completely locks and doesn't proceed at seemingly random times. I couldn't think of a sane way to detect this, killing containers seemed easier and has worked reliably so far. I'm definitely open to hearing suggestions though.

That aside though Tini looks like a good starting point for containers like mine anyways. So thanks, it does seem helpful!

It runs out of entropy? Not sure if this applies to containers, but had something like this happen in a virtual machine we used for builds (Jenkins). I made the mistake of instantiating a cryptographically secure PRNG during build time; the initialization needed entropy to seed the CSPRNG with, and this would lock up the whole virtual machine in the middle of the build. It seems that headless VMs have no possible source of entropy, unless explicitly configured to use host's source. Maybe something similar happens with containers?